1. Field of the Invention
The present invention relates to a multilayer ceramics substrate having a surface conductor made of a sintered metal and the like on a surface.
2. Related Background Art
While substrates for mounting electronic devices have widely been in use in the fields of electronic appliances and the like, multilayer ceramics substrates have recently been proposed and put into practical use as a highly reliable substrate responding to demands for the electronic appliances to reduce their size and weight and diversify their functions. A multilayer ceramics substrate is constructed by laminating a plurality of ceramics substrate layers and enables high-density mounting by integrating surface conductors, inner conductors, electronic elements, and the like with the ceramics substrate layers.
When a surface conductor formed on an outer surface of the multilayer ceramics substrate is made of a sintered metal, a conductor paste employed is typically doped with glass frit. Doping with glass frit is effective in securing the bonding strength between the surface conductor made of the sintered metal and the ceramics substrate layer. For example, using a conductor paste containing an Ag powder (conductive material) and glass frit as a conductive paste for forming the surface conductor can secure the adhesion between the surface conductor and ceramics substrate layer to a certain extent.
For keeping the electric resistance of the surface conductor at a low value and making its solder wettability and platability favorable, the doping amount of glass fit has its limit, so that sufficient results may not always be obtained by doping with glass frit alone. Therefore, various studies have been under way in order to improve the bonding strength of the surface conductor, and it has been proposed to secure the bonding strength of the surface conductor, for example, by forming an undercoat layer or the like (see, for example, Japanese Patent Application Laid-Open Nos. HEI 6-237081 and 9-221375).
Namely, the invention described in Japanese Patent Application Laid-Open No. HEI 6-237081 forms a glass layer at the interface between a glass ceramics layer and a conductor lead on the uppermost layer by printing a glass paste, thereby attaining a high bonding strength. Similarly, the invention described in Japanese Patent Application Laid-Open No. HEI 9-221375 forms an undercoat layer containing predetermined glass and alumina between a ceramics substrate layer and a surface conductor layer, so as to secure the adhesion between the substrate and surface conductor layer.